1. Field
The present disclosure relates to a method and a system for minimizing cross-process non-uniformities in solid and heavy shadow regions of printed documents.
2. Description of Related Art
An electrophotographic, or xerographic, image printing system employs an image bearing surface, which is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the image bearing surface is exposed to a light image of an original document being reproduced. Exposure of the charged image bearing surface selectively dissipates the charge thereon in the irradiated areas to record an electrostatic latent image on the image bearing surface corresponding to the image contained within the original document. The location of the electrical charge forming the latent image is usually optically controlled. More specifically, in a digital xerographic system, the formation of the latent image is controlled by a raster output scanning device, usually a laser or LED source.
After the electrostatic latent image is recorded on the image bearing surface, the latent image is developed by bringing a developer material into contact therewith. Generally, the electrostatic latent image is developed with dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto. However, a liquid developer material may be used as well. The toner particles are attracted to the latent image, forming a visible powder image on the image bearing surface. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to a media, such as sheets, paper or other substrate sheets, using pressure and heat to fuse the toner image to the media to form a print.
The image printing system generally has two important dimensions: a process (or a slow scan) direction and a cross-process (or a fast scan) direction. The direction in which an image bearing surface moves is referred to as the process (or tie slow scan) direction, and the direction perpendicular to the process (or the slow scan) direction is referred to as the cross-process (or the fast scan) direction.
Electrophotographic image printing systems of this type may produce color prints using a plurality of stations. Each station has a charging device for charging the image bearing surface, an exposing device for selectively illuminating the charged portions of the image bearings surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the respective electrostatic latent image. The images are developed, at least partially in superimposed registration with one another, to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a media. The transferred multicolor image is then permanently fused to the media forming the color print.
Improving uniformity in images is desirable for high quality rendering. An array sensor (e.g., a Full Width Array (FWA)) enables scanning the developed images to measure a wide variety of image defects that might occur in the xerographic process. The array sensor may be used to detect both non-uniformities in the cross-process direction and process direction (i.e., streaks and bands, respectively).
“Streaks” as used herein are uniformity variations in the cross-process direction, at all spatial frequencies (i.e., “narrow” streaks as well as “wide” streaks including variations along the lateral side of the image printing system), and at all area coverage levels. Streaks are a major image quality defect for both image-on-image (IOI) and Intermediate Belt Transfer (IBT) Tandem xerography. Streaks are primarily one-dimensional visible defects in the image that run parallel to the process direction (i.e., the slow-scan direction). In a uniform gray level patch, streaks may appear as a variation in reflectance. As used herein, “gray” refers to the area coverage value of any single color separation layer, whether the toner is black, cyan, magenta, yellow, or some other color. In a color xerographic machine, streaks in single color separations that may be unobjectionable alone can cause an undesirable visible color shift for overlaid colors.
Image printing technologies may contain several sources of streaks, which can sometimes be difficult to control via image printing system design or image printing system optimization. Streaks may be caused by “non-ideal” responses of xerographic components in the marking engine of the image printing system. The source of these artifacts may be found in contamination of the development wires, photoreceptor (P/R) non-uniformities, fuser non-uniformities, charge device contamination, etc. Streaks may also be caused by non-uniformity of the raster output scanning device spot-size or intensity variations.
As shown in FIG. 1, a measured reflectance profile of a single color, uniform test image generated by the image printing system is shown. The reflectance profile is generated by measuring the reflectivity of the image in the cross-process direction. The measured reflectance profile illustrates streaks as undesired variations in cross-process reflectance in the test image. It is well known by practitioners of the art that a relationship exists between luminance, as described, for example, by CIELAB L*, and reflectance. Thus if a printed page of the test target whose reflectance is depicted in FIG. 1 were measured for its L* profile, it would look similar to FIG. 1 with a change of scale. A desired reflectance profile for such a test image would be flat. Similarly, a desired L* profile for the test image would also be flat.
Non-uniformities in the cross-process direction (e.g., streaks) in halftones and solid regions may be very problematic to mitigate using spatial exposure modifications since many Raster Output Scanner (ROS) devices only support modulations in the cross-process direction at low spatial frequency. In other words, the ROS devices do not have the capability to adjust intensity fast enough to compensate at medium to high frequency (i.e., very low frequency intensity adjustment is achievable). This makes compensating higher spatial frequency non-uniformities in the cross-process direction, such as charge device non-uniformities, extremely difficult, using the exposure intensity actuator. Also, many ROS devices also do not support adjusting intensity on a pixel-by-pixel basis in the cross-process (i.e., fast-scan) direction. This makes compensating for higher spatial frequency cross-process non-uniformity through spatial exposure modulation using such devices nearly impossible.
Image Based Control (IBC) techniques enable compensation of streaks by sensing: the uniformity across the process of the image printing system and spatially actuating either the exposure or a Spatially Varying Tone Reproduction Curve (STRC).
For example, U.S. Pat. No. 6,760,056; U.S. Patent Application Publication No. 2006/0209101; and U.S. Patent Ser. No. 61/056,754 filed on May 28, 2008 describe a streak correction system that uses an in situ monochrome sensing full width array sensor to sense streak non-uniformities and uses spatially varying Tone Reproduction Curves (TRC) as actuators to correct for the streak non-uniformities. The streak correction system described in the above-mentioned references utilizes image-based correction to compensate for streak non-uniformities. However, the streak correction system described in the above mentioned references cannot correct streaks occurring in solid regions (i.e. because every pixel in the digital image is already “on” for a solid region).
U.S. Patent Application Publication No. 2006/0001911 describes another streak correction system that uses an offline scanner to determine the streak non-uniformities and uses cross process, or fast scan direction, adjustment of the ROS exposure to correct for the non-uniformities. Smile correction is a well known technique used in polygon ROS systems to correct for aerial image illumination non-uniformity across the scan line. It is accomplished by “calibrating” the ROS by measuring the illumination level at several points (for example, 20) along the ROS scan line. Smile correction is a standard technique for correcting low spatial frequency non-uniformity in the fast scan direction using the ROS as the actuator. Many types of ROS devices do not allow for pixel-by-pixel adjustment of the exposure intensity. Thus, there are substantial limitations to the spatial frequencies for which these ROS devices alone can be used to compensate print non-uniformities in the cross-process direction. In addition, depending on the xerographic setup with respect to the slope of the photo-induced discharge characteristic (PIDC) curve, the ROS may not have much actuator authority in the solid and shadow regions.
U.S. patent Ser. No. 12/112,618 filed on Apr. 30, 2008 describes the combination of ROS actuation and spatially varying TRC actuation for streaks compensation. However, the exposure actuation is used for low spatial frequency non-uniformities (which often have a larger amplitude) and the spatially varying TRC actuator is used for high spatial frequency non-uniformities (which usually have a smaller amplitude). The streak correction scheme described in this reference does not propose any special processing to compensate for streaks in solid and heavy shadow regions.
Therefore, the streak compensation techniques described above have opportunities for improvement. As noted above, the spatial actuation of the exposure often has limitations in spatial bandwidth (i.e., it is good for low spatial frequencies only—“smile” correction) and may have limited actuator authority in the heavy shadow and solid regions due to xerographic setup on a more saturated portion of a photo-induced discharge characteristic (PIDC) curve. Spatial TRC actuation exhibits wide spatial bandwidth, however it has limited actuator authority in the heavy shadow regions and only unidirectional authority at the solid. This type of correction, using the digital image and spatial TRCs as actuators, cannot adequately correct streaks that occur in solid and heavy shadow regions. In other words, by using only the digital image as the actuator, it is impossible to make the digital image to be “darker” than every pixel on.
Also, bandwidth limitations on many current exposure technologies do not allow modification of the exposure intensity in the cross-process direction at sufficiently high spatial frequencies to correct for many streak artifacts.
Further, the image based control streak correction methods discussed in above references can only make a solid “lighter” rather than making it “darker”. Therefore, either the solid is lightened to achieve uniformity at the cost of a greatly reduced gamut (i.e., solid regions are brought to a uniform lightened state), or the solid regions are not made uniform (i.e., but the gamut is not intentionally affected). Gamut, in color reproduction, refers to a subset of colors that can be accurately represented in a given circumstance. The color gamut could still be affected by the defect. In other words, if there are streaks the macroscopic effect could be a substantially lighter output, i.e., reduced gamut. The inventors in the present disclosure propose maintaining (and in most cases increasing) the gamut (i.e., by maintaining the “darkness” of the solid regions, while also improving solid area uniformity).
Thus, the present disclosure provides improvements in streak compensation techniques that address the streaks in solids and shadow regions at both the high spatial frequency and low spatial frequency content, while maintaining the accessible color gamut of the print process across the full inboard-to-outboard process width of the printing system.